With development of communications technologies, antennas of different frequency bands need to be integrated into one antenna. When sharing a same dipole, antennas of different frequency bands each need to have an independent downtilt adjustment function. Therefore, after the antennas are each provided with a phase shifter for downtilt adjustment, a combiner needs to be used to synthesize signals of the antennas onto the same dipole. Usually, because two frequency bands may be relatively close, each filter that forms the combiner needs to have a steep roll-off feature.
Usually, a filter with a steep roll-off feature is a band-stop filter, and an advantage of the band-stop filter is that a frequency of a passband is very close to a frequency of a stopband, so that a requirement for suppressing at least 30 dB at tens of MHz or ten-odd MHz outside the passband can be satisfied. Therefore, a combiner is usually formed by a plurality of band-stop filters. As shown in FIG. 1, on the left of a combination port, a passband frequency of a band-stop filter is lower than a stopband frequency, and on the right of the combination port, a passband frequency of a band-stop filter is higher than a stopband frequency. However, such a combiner has a disadvantage that in the band-stop filter whose passband frequency is higher than the stopband frequency, a transmission line of a signal channel between two resonant cavities needs to be designed to be ¼ longer than a resonant frequency wavelength. This increases a volume of the combiner.